Most conventional coating resins are insoluble in water. Therefore, in general practice, they have been dissolved in a suitable organic solvent or dispersed in water with the aid of an emulsifying agent or surfactant in order to provide a coating composition suitable for application to a substrate surface. A serious disadvantage of organic solvent solutions is that they are potentially toxic, flammable and environmental pollutants. Water-reducible coatings greatly reduce the magnitude of these problems. For this reason, water-based paints are currently being used as a replacement for oil-based paints in many applications.
Various water-reducible coating resins, such as the one described in U.S. Pat. No. 4,474,926, have been developed. Water-reducible coatings that utilize such resins have been developed for a variety of purposes and have been widely accepted in many applications such as highway striping paint.
U.S. Pat. No. 4,968,741 describes a coating for metal substrates which provides improved corrosion and rust resistance. Such coatings are of the water-reducible type and can be beneficially utilized in the automotive industry and other applications where good rust resistance is needed. For instance, such coatings are excellent for coating bridges and other outdoor metal structures.
It is also critical for coatings made with water-reducible coating formulations to offer the desired combination of physical and chemical properties. For instance, in many applications, it is important for the coating to exhibit excellent flexibility, excellent ultra-violet light resistance and excellent solvent resistance. In applications which involve metal substrates, outstanding corrosion and rust-resistance is normally also sought.
For purposes of this patent application, an aqueous coating system is considered to be a colloidal dispersion of a resin in water which can be reduced by the addition of water and which forms a durable coating when applied to a substrate surface. The term aqueous coating system is used herein interchangeably with the term water-reducible coating. Other names which are sometimes applied to water-reducible coatings are water-born, water-solubilized and water-dilutable.